Mix butter and sugars, until light and fluffy. Add vanilla extract, eggs and mix. Add dry ingredients and mix until just combined. Do not over mix. Fold in Chocolate chips.
Spread 2/3 of the cookie dough into your prepared pan; batter will be sticky, so either wet your fingers with water or use a piece of parchment paper to spread an even layer
Add dollops of the marshmallow fluff on top of the cookie layer, distributing them all over the pan, then spread them as evenly as possible (don't worry if not everything is covered!). Using your fingers, crumble the remaining dough on top of the marshmallow fluff layer, you want some marshmallow fluff picking out between the cookie crumbles. Add a few chocolate chips on top if desired
1 Cup(127g) marshmallow fluff
Bake for 22-25 minutes, or until a toothpick inserted in the middle comes out almost clean. Let cool completely over a wire rack. Cut in 24 bars and enjoy!
Notes
* Take butter and eggs out of the fridge 30-60 minutes before using, depending on the temperature at the time* After adding flour, do not over beat the dough, or you risk it becoming cakey